High Resolution X-ray Diffraction Scans Research Articles

Semimagnetic II–VI semiconductor resonant tunneling diodes characterized by high-resolution X-ray diffraction

We show that important layer parameters of the double barrier active region in semimagnetic II–VI semiconductor resonant tunneling diodes (RTDs) can be accurately determined from high-resolution X-ray diffraction (HR-XRD) scans of complete structures. This is remarkable, since the scattering volume of the typically only few nm thin double barrier region is low compared to the thick, complex contact and buffer layers it is embedded in. The parameters obtained by HR-XRD of several RTD structures are correlated with current–voltage characteristics, finding good agreement with the expected exponential dependence of the off-resonance tunneling current on the measured barrier thickness and height. Thus direct and nondestructive determination and optimization of the active region properties are possible for such II–VI spintronic devices.

Structural, electrical and optical characterization of InGaN layers grown by MOVPE

We present a study on n-type ternary InGaN layers grown by atmospheric pressure metalorganic vapour phase epitaxy (MOVPE) on GaN template/(0001) sapphire substrate. An investigation of the different growth conditions on n-type Inx Ga1 − x N (x = 0.06 − 0.135) alloys was done for a series of five samples. The structural, electrical and optical properties were characterized by high resolution x-ray diffraction (HRXRD), Hall effect and photoluminescence (PL). Experimental results showed that different growth conditions, namely substrate rotation (SR) and change of total H2 flow (THF), strongly affect the properties of InGaN layers. This case can be clearly observed from the analytical results. When the SR speed decreased, the HRXRD scan peak of the samples shifted along a higher angle. Therefore, increasing the SR speed changed important structural properties of InGaN alloys such as peak broadening, values of strain, lattice parameters and defects including tilt, twist and dislocation density. From PL results it is observed that the growth conditions can be changed to control the emission wavelength and it is possible to shift the emission wavelength towards the green. Hall effect measurement has shown that the resistivity of the samples changes dramatically when THF changes.

Quantum Dot Spectral Tuning of Multijunction III-V Solar Cells

AbstractImproving the production of photocurrent in the middle junction of a InGaP/Ga(In)As/Ge triple-junction solar cells (TJSC) can improve the overall conversion efficiency of cell. One possible method to improve the middle junction photocurrent is inserting a quantum dot (QD) superlattice (SL) into the stack. It has been predicted that QD-SL enhanced TJSCs have an efficiency ceiling of 47% under a one-sun AM0 illumination spectrum. Additionally, QD array enhanced GaAs cells have the added benefit of possible intermediate band effects, anisotropic absorption and enhanced radiation tolerance. Embedding InAs quantum dots (QDs) in a single junction GaAs solar cell can increase sub-GaAs bandgap photocurrent generation. This method has been shown to improve the short circuit current density (Jsc) of single junction cells under simulated 1 sun air mass zero (AM0) illumination. However, the increase in strain due to the InAs QD self-assembly may cause defects that reduce the minority carrier lifetime resulting in losses in the cell open circuit voltage (Voc) on the order of 300-500 mV. The introduction of strain compensation (SC) layers into the superlattice (SL) structure of a QD solar cell has previously been shown to improve the device performance, including the partial recovery of Voc. Strain compensation can be used effectively to balance the residual strain, impede dislocation formation, and improve the solar cell characteristics. The effect of GaP strain compensation on the solar cell electrical and material properties was investigated. High resolution X-ray diffraction (HRXRD) scans along the symmetric (004) Bragg peak show weak intensity and wide FWHM at the zero order SL peak (SL0) for non-SC samples. Optimum SC thickness was theoretically determined using a zero in plane stress method and experimentally verified using HRXRD. Optimal strain compensation was then used to increase the QD SL stacking from 5x to 10x and 20x. Use of SC resulted in shifting of the SL0 peak toward the substrate peak as well as reduced FWHM and improved SL peak definition. Kinematical diffraction modeling of the QD structures using numerical simulation indicated this peak shift resulted from reduced overall strain in the SL stack up to 5ML of SC. The material quality improvement in the SC QD solar cells was manifested in an improved spectral response and Jsc. The optoelectronic results for GaAs solar cells with QD SL’s demonstrate a strong dependence on GaP SC layer thickness. In addition, comparison of multi junction (MJ) solar cells which incorporate the SC QD SL’s demonstrate the utility of additional sub-GaAs bandgap current contribution as a tool for additional current-matching optimization in MJ solar cells.

A new type of structural defects in CdZnSe/ZnSe heterostructures

The changes of structural and photoluminescence (PL) characteristics of MBE-grown CdZnSe/ZnSe single quantum well (QW) structures caused by Cd/Zn interdiffusion were studied by high-resolution X-ray diffraction (HRXRD) and low-temperature PL methods. The PL investigations showed that the increase of Cd content in the QW resulted in the increase of the depth of potential fluctuations in the QW as well as in the decrease of ZnSe cap layer band gap (up to several meV). The HRXRD scans as well as reciprocal space maps measured for symmetrical and asymmetrical diffractions revealed the formation of CdZnSe layers near the QW/ZnSe interface with Cd content of order of several percents. It is found that in some samples, the Cd profile in these layers is rather smooth while in others it is quite abrupt. In the latter case, the partial strain relaxation can occur in these layers.

Quantification of In clustering in InGaN∕GaN multi-quantum-wells by analysis of x-ray diffraction data

The authors developed a method of quantification of lateral indium content fluctuations in InGaN∕GaN multi-quantum-wells (MQWs) grown on bulk GaN substrates. The method is based on comparison of intensities of the substrate and the MQW zeroth order peaks in 2θ-ω high resolution x-ray diffraction scans. They use a simulation program based on the kinematical theory of diffraction to fit simulated curves to the measured ones. The program uses the indium composition fluctuation amplitude as a fitting parameter.

Athermal annealing of Mg-implanted GaAs

High-resolution X-ray diffraction scans and electrical resistivity measurements were performed on Mg-implanted GaAs exposed to laser shock annealing on both the implanted and the unimplanted surfaces of the wafer. Measurements on the sample that was subjected to a laser pulse exposure on the implanted side indicate that annealing had occurred within 3 mm of the center of the laser-exposed spot. X-ray scans and the topographs indicate the propagation of a mechanical wave propagating in the radial direction from the laser-exposed spot. No evidence of annealing, however, was observed at any position on the Mg-implanted GaAs sample that was subjected to a laser pulse exposure on the unimplanted side. The Mg implant did not show any redistribution for the laser-shocked annealing, in contrast to the conventional rapid thermal annealing.

Damage accumulation in Si during high-dose self-ion implantation

Accumulation and annealing of damage in Si implanted with self-ions to high doses were investigated using a combination of grazing incidence diffuse x-ray scattering, high-resolution x-ray diffraction scans, and transmission electron microscopy. During implantation at 100°C, small vacancy and interstitial clusters formed at low doses, but their concentrations saturated after a dose of ≈3×1014cm−2. The concentration of Frenkel defects at this stage of the implantation was ≈1×10−3. At doses above 1×1015cm−2, the concentration of implanted interstitial atoms began to exceed the Frenkel pair concentration, causing the interstitial clusters to grow, and by ≈3×1015cm−2, these clusters formed dislocation loops. Kinematical analysis of the rocking curves illustrated that at doses above 1×1015cm−2 the “plus one” model was well obeyed, with one interstitial atom being added to the dislocation loops for every implanted Si atom. Measurements of Huang scattering during isochronal annealing showed that annealing was substantial below 700°C for the specimens irradiated to lower doses, but that little annealing occurred in the other samples owing to the large imbalance between interstitial and vacancy defects. Between 700 and 900°C a large increase in the size of the interstitial clusters was observed, particularly in the low-dose samples. Above 900°C, the interstitial clusters annealed.

GaN epitaxy on thermally treated c-plane bulk ZnO substrates with O and Zn faces

ZnO is considered as a promising substrate for GaN epitaxy because of stacking match and close lattice match to GaN. Traditionally, however, it suffered from poor surface preparation which hampered epitaxial growth in general and GaN in particular. In this work, ZnO substrates with atomically flat and terrace-like features were attained by annealing at high temperature in air. GaN epitaxial layers on such thermally treated basal plane ZnO with Zn and O polarity have been grown by molecular beam epitaxy, and two-dimensional growth mode was achieved as indicated by reflection high-energy electron diffraction. We observed well-resolved ZnO and GaN peaks in the high-resolution x-ray diffraction scans, with no Ga2ZnO4 phase detectable. Low-temperature photoluminescence results indicate that high-quality GaN can be achieved on both O- and Zn-face ZnO.

Nanocavities in He implanted InP

The formation of nanocavities in InP(001) by room-temperature He implantation and subsequent thermal annealing was studied using a combination of high-resolution x-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM) analyses. The nanocavities size and depth distributions were measured as a function of He ion dose φHe (1×1016 to 9×1016 cm−2) and ion energy E (25 to 70 keV), as well as annealing temperature Ta (600 to 750 °C) and time ta (5 to 25 min). HRXRD scans from annealed samples indicate an expansion of the InP lattice, contrary to what is usually observed following heavy-ion implantation. The critical φHe and Ta values for the formation of nanocavities were found by XTEM analysis to be between 1 and 2×1016 cm−2 and between 600 and 620 °C, respectively. Cavities of diameter 4–50 nm with {110}, {101}, and {001} facets were obtained. Increasing Ta and ta resulted in larger cavities and increasing φHe produced a larger number of cavities. Furthermore we find that nanocavities are metastable as their size first increases with annealing temperature and time but then decreases until they disappear for ta>25 min at Ta=640 °C or ta>10 min at Ta=750 °C. Results are compared with similar work carried out on He-implanted silicon and differences between the two materials are explained in terms of defect diffusivity and surface energy, higher diffusivity enhancing cavity collapse and lower surface energy enhancing cavity growth.

Nanobumps in InxAl1-xN/AlN/Sapphire System:A New Kind of Quantum Dots?

AbstractWe have investigated InxAl1-xN layers grown by plasma source molecular beam epitaxy (PSMBE) on c-plane sapphire (0001) substrates with thin (about 100 Å) buffer layers of AlN. The value of x varied from 0 to 1. For all these layers, high resolution X-ray diffraction scans (XRD) show no indication of face segregation, while the atomic force microscope (AFM) images of the structures demonstrate a large number of nanobumps. Our spectroscopic measurements (UV/VIS optical absorption and reflection spectroscopy) of these bumpy structures indicate additional peaks that can be only associated with additional energy levels. We ascribe these levels to electrons, which are confined and quantized near the nanobump tips by strong piezoelectric field caused mainly by biaxial strain of the AlN layer. The calculated energies of these quantized states are in a good agreement with the spectroscopic data.

A New Approach for Determining Epilayer Strain Relaxation and Composition Through High Resolution X-Ray Diffraction

ABSTRACTWe developed a new method of determining epilayer relaxation (along one direction) and composition using a symmetric and any single asymmetric high resolution x-ray diffraction scan. The previous use of small angle approximations can be very detrimental to calculated results and should be avoided. This new method does not employ small angle approximations or first order Taylor approximations, producing accurate results. The effect of x-ray geometry (glancing incident versus glancing exit) on the analysis of epilayer composition and strain is also reviewed. It is also shown that the glancing exit geometry is generally less susceptible to experimental error.

Slits and high-resolution X-ray diffraction

It is shown that, with proper use of an anti-scatter slit in front of the detector, the signal-to-noise ratio of a symmetric high-resolution X-ray diffraction scan can be improved by a factor of ten. By the use of an asymmetric reflection with a high angle of incidence on the sample, the size of the diffracted beam can be reduced sufficiently to allow for two-dimensional reciprocal-space scans with a simple slit instead of a crystal assembly in front of the detector for enhanced resolution. By selection of the proper reflection, a resolution can be chosen that suits the application. Examples include a partially relaxed SiGe multilayer and a periodic surface grating.

Optimal epilayer thickness for InxGa1−xAs and InyAl1−yAs composition measurement by high-resolution x-ray diffraction

The composition of InxGa1−xAs and InyAl1−yAs epitaxial layers on InP substrates can be measured by high-resolution x-ray diffraction (HRXRD) in many cases. If layers are too thick, however, substantial lattice relaxation will occur, requiring multiple asymmetric scans to determine composition. If layers are too thin, they will not produce a distinct Bragg peak. Based upon measurements of both coherent and relaxed layers as well as simulations, we have determined the range of epilayer thickness over which a single HRXRD scan yields the composition of InxGa1−xAs and InyAl1−yAs layers to within 1%. Calibration layers grown within this range allow fast and accurate characterization.

Type II superlattices for infrared detectors and devices

Superlattices consisting of combinations of III-V semiconductors with type II band alignments are of interest for infrared applications because their energy gaps can be made smaller than those of any 'natural' III-V compounds. Specifically, it has been demonstrated that both InSb/InAsxSb1-x superlattices and Ga1-xInxSb/InAs superlattices can possess energy gaps in the 8-14 mu m range. The efforts have focused on the Ga1-xInxSb/InAs system because of its extreme broken gap band alignment, which results in narrow energy gaps for very short superlattice periods. The authors report the use of in situ chemical doping of Ga1-xInxSb/InAs superlattices to fabricate p-n photodiodes. These diodes display a clear photovoltaic response with a threshold near 12 mu m. They have also attained outstanding structural quality in Ga1-xInxSb/InAs superlattices grown on radiatively heated GaSb substrates. Cross-sectional transmission electron microscope images of these superlattices display no dislocations, while high resolution X-ray diffraction scans reveal sharp high-order superlattice satellites and strong Pendellosung fringes.